1. Field of the Invention
The present invention relates to a tag detecting system for detecting a movement of an object mounting an RFID tag, a moving object detecting method, and an entrance/exit management system which is realized by the moving object detecting method.
2. Description of Related Art
Recently, an RFID tag has been widely used in buried in an IC card, or used in management systems such as an article management system and an entrance/exit management system of persons. Among the RFID tags, particularly, a semi-passive type or an active type of the RFID tag that transmits waves by a cell energy incorporated in the RFID tag, is preferably used in the entrance/exit management system of persons. The semi-passive type or the active type of the RFID tag has a quite ampler communication range (for example, 1 m-10 m) than a passive type of the RFID tag buried in the IC card.
For example, an art of the entrance/exit management system for accurately managing an entrance to and exit from a facility is disclosed. According to the art, the RFID tag that periodically transmits an ID (hereinafter, called an active tag) is used in the management system. Further, according to the art, by relatively shifting directions of mono pole direction antennas arranged at both right and left sides of an entrance/exit gate, if a person having an active tag passes through the gate, entrance and exit directions of the person can be accurately detected by order in which the respective antennas detect the active tag (refer to Japanese Laid-Open Patent Publication No. 2002-334382, paragraphs 0009 to 0015, and FIGS. 1 and 2). Additionally, another art on the entrance/exit management system is disclosed, which detects a position of a person having the RFID tag that transmits an ID (hereinafter, called a semi-passive tag), by arranging infrared transmitters which have different position IDs and antennas which form high frequency magnetic fields at entrance/exit doors, and by receiving signals from the antennas. According to the art, the position of a specific person is determined by the position ID which is transmitted from the respective antennas and the ID of the RFID tag held by the person. Therefore, only the specific person can be discriminated from a plurality of persons having the RFID tags who enter or exit at almost the same time. For example, see Japanese Laid-Open Patent Publication No. 2003-21679 (paragraphs 0009-0015, and FIGS. 1, 2 and 3).
However, a system for detecting the entrance to and the exit from the facility by the active tag, is liable to be influenced by a wave reflector and an obstacle surrounding the system. Further, intensity of a transmitted wave changes by a carrying method of the active tag. These drawbacks reduce accuracy for detecting a position and make the system difficult used outside. Moreover, in case of an infrared transmitter method using the semi-passive tag which can be used outside, the tag needs to be mounted on an outer surface of a person in order to easily receive infrared rays. Hereby, a problem is that the method is liable to be influenced by an external light. In case of a high-frequency magnetic field method, since the transmitting range thereof extends in a spherical shape, a problem is that a tag detection range becomes too broad. As a result, after a person (or an object) having the RFID tag is certified and passes through a gate, if a person having no RFID tag adjacently passes through the gate (so called a piggyback), the conventional method has no means to detect the piggyback because the detection range is unstable or too broad. An art that distinguishes a person having a tag from a person having no tag is needed, which detects the person (object) having the tag in a stable and narrow range by using a radio wave such as a high-frequency magnetic field, so that the piggyback is exactly detected by having a person mount the tag, for example, in a pocket.